Go Down

Topic: PWM Output Causes ADC Spikes (Read 181 times) previous topic - next topic

randomvibe

Jul 07, 2015, 07:59 pm Last Edit: Jul 07, 2015, 08:00 pm by randomvibe
My project uses ADCs at 12bit resolution to sense accels and rate gyros, and also uses hardware PWM to control a motor driver.  PWM uses: pins 6 & 7 at 10khz, 16 bit resolution.  An optoisolator is put in between the Arduino Due and motor driver.

The problem is that the PWM causes voltage spikes in the ADC readings.  At first, I suspected the rate gyros themselves.   But it turned out to be the PWM signal.  When I comment out the PWM calls, the spikes disappear.  What can be done about this?  I read about decoupling capacitors, but not sure where they need to be and what values to use. 

Magician

Optocoupler may take 10-20 mA, post your drawings.
http://coolarduino.blogspot.ca

randomvibe

It's definitely the optoisolator.  I put in the PWM calls back in the program, and disconnected the optoisolator, and the ADC spikes disappear.

Shortening the PWM wires from 3 feet to 12 inches did not fix the issue.  Adding decoupling capacitors (47 nf per the specs) did not fix the issue.

Putting in a 220 ohm resistor between the PWM pin and the optoisolator input seems to help.  I also tried using other Arduino Due ADC pins, and that also seems to help.  Unfortunately, my project uses 7 ADCs, so a few a them inherit the spikes.

I use the HCPL9030 optoisolator for it's speed.  I tried another version (HCPL2631), but that makes the problem worse.  See attachment.

Has anyone seen similar issues regarding ADC spiking with hardware PWM connected to a load?



MorganS

You don't already have a resistor? That could be a problem. Show your schematic. A photo of a hand drawing is OK.
GoForSmoke: "What GShield? You never mentioned a shield."

Go Up